Suspensions for trucks and the like are very well known in the art.
Indeed, a conventional vehicle suspension for connecting an axle to the structural frame, or chassis, includes a combination of springs and shock absorbers for absorbing, isolating and dampening the movements transmitted between the axle and the chassis. Typically, a vehicle such as a truck will include a front axle supporting a pair of front wheels and at least one rear axle supporting a pair of rear wheels. A second (rear tandem) and even third (rear tridem) rear axle can be provided to increase the load capacity of a given truck.
It is also known in the art to provide a second front axle to further increase the load capacity of a truck. Such a front suspension is called a front tandem suspension.
Canadian Patent No. 2,070,859, issued Jan. 10, 1995 to Simard and titled “Tandem Axle Suspension for Vehicle”, describes a front suspension for a truck or semi-trailer including front and rear tandem axles connected to a vehicle chassis by first and second leaf springs, respectively. In order to distribute the load evenly between the front and rear axles, an equalizer beam is provided rockably mounted onto the chassis and connected at one end to the rear extremity of the first leaf spring, and at the other end to the front extremity of the second leaf spring.
It is also known to use air springs in conjunction with, or in place of, conventional springs in order to improve ride comfort.
U.S. Pat. No. 6,382,659, issued May 7, 2002 to Simard and titled “Load Distributing Tandem Suspension Assembly”, describes a tandem front suspension similar to that described above, with the addition of an air spring and shock absorber for supporting the second front axle in conjunction with the second leaf spring.
A drawback associated with tandem suspensions is that they decrease the traction of each wheel. While a tandem suspension allows a truck to bear more weight by providing an additional front axle with which to further support the truck's load, this increased capacity comes with a price. As the load is spread between the various axles, and wheels, so is the traction. This is especially apparent and detrimental when the truck is unloaded and what little weight remains is distributed between the plurality of axles and wheels. As a result, single front axle trucks must often be used for servicing sites which require off-road travel as tandem axle trucks cannot generate enough traction to navigate off-road when unloaded.
Known in the art are the following U.S. patents which disclose prior-art suspensions.
U.S. Pat. No. 3,285,621, issued Nov. 15, 1966 to Turner, describes a vehicle suspension having a first axle mounted to a chassis by a conventional leaf spring and a second axle mounted to the chassis by both a leaf spring and air spring. A pneumatic system inflates the air spring, but can also deflate the air spring in order to raise the second axle and lift its wheels off the road surface.
U.S. Pat. No. 3,762,487, issued Oct. 2, 1973 to Bilas, describes a tandem vehicle suspension comprising first and second axles. Connecting the axles to the vehicle's chassis are a pair of air springs and a shared leaf spring. The air springs are aligned with respective axles, while the shared leaf spring is mounted to the chassis at its midpoint and engages the axles at its extremities. The air springs are connected to a pneumatic system which is operable to deflate one of the air springs in order to raise an axle and lift its wheels off the road surface.
U.S. Pat. No. 6,364,340, issued Apr. 2, 2002 to Taylor, describes a similar tandem vehicle suspension comprising a pair of axles mounted at either end of a shared leaf spring. An air spring is provided between the chassis and one of the axles and is operable to be inflated such that the opposing axle and wheels are raised from the road surface.
A drawback of these tandem suspensions is that in enabling the unloading of one of the tandem axles, the ride comfort and quality can be significantly diminished. This is especially true when the tandem are steerable axles, i.e. axles that are supporting steerable wheels rather than fixed wheels, in contrast with the patents of Turner, Bilas and Taylor which primarily provide tandem suspensions for non-steered axles.
In view of the above, there remains a need for a tandem suspension which provides increased ride comfort for a user and overcomes the aforementioned limitations of the prior art while exploiting the advantages of a tandem air spring suspension. There further remains a need for a suspension which combines the increased load capacity of a front tandem axle suspension without the aforementioned loss of traction.
Also known in the art are the following patents and published applications which also describe tandem suspensions and the like: U.S. Pat. Nos. 3,063,732, 3,233,915, 3,902,734, 4,619,467, 4,676,523, 4,966,387, 5,271,638, 5,354,091, 5,615,906, 5,873,581, 6,224,074, 6,276,710, 6,460,872, 6,857,647, US2004/0080136, US2005/0263986, and US 2006/0208464.